Long-term storage of potable water is necessary for certain missions carried out beyond the Earth's stratosphere. Spacecraft such as the Space Shuttle or the Orion are or will be used to transport astronauts, crew and other personnel from Earth to orbiting space stations, the moon or Mars. A source of potable water is needed for those aboard the spacecraft. While water recycling systems exist, they can be complicated, subject to failure and may not be able to provide adequate quantities of water on demand. Thus, in some circumstances, a source of potable water in a storage tank or other vessel may be used to carry the water needed during the course of the mission or provided in addition to a water recycling system. In these cases, water is typically charged to the tank or vessel on the ground, shortly before launch.
Long-term storage of potable water on a spacecraft is desirable in situations where a spacecraft will be unable to take on additional supplies of potable water during its mission or where launch is delayed (eliminating the need to empty the tank and recharge additional potable water). For example, potable water may need to be stored on a spacecraft for many weeks during travel to Mars or when the spacecraft has an extended docking period at a space station and potable water is required for the return trip to Earth. In some cases, the potable water may remain on the spacecraft until a return voyage which may be many days, weeks or months after the initial departure.
Silver ions are known to exhibit a toxic effect on bacteria, viruses, algae and fungi, similar to some heavy metals like lead and mercury, but without the high toxicity to humans normally associated with those heavy metals. Silver biocides have been used to preserve sources of potable water in vessels made of plastic and other polymers. Silver ions at levels between about 0.02 parts per million (ppm) and about 0.4 ppm are able to maintain water quality at or below microbial limits of approximately 50 CFU/mL (colony forming units per milliliter). However, plastic vessels are typically not robust enough to withstand spacecraft launch and landing conditions. Metallic vessels are strong enough for use on spacecraft, but studies have shown that silver ions are rapidly depleted in vessels having metallic surfaces. Initial concentrations of silver ions present in potable water up to 0.4 ppm can be reduced below detection limits (and effective biocidal levels) within 28 days of exposure to some metallic surfaces when the ratio of metallic surface area to water volume approximates the ratio expected in a water storage tank. The depletion time may be shortened when the surface area to volume ratio is higher, as in a tube. In order for metallic vessels to be useful for long-term storage of potable water containing silver biocides, depletion of silver ions caused by metallic surfaces must be reduced or eliminated.